Switching power supplies are used to provide power in numerous products such as cell phones, camera, PDAs (Personal Digital Assistants), calculators, portable computers and similar types of electronic equipment. Such switching power supplies are quite complex and use numerous components to provide a number of precisely regulated output voltages to power the various integrated circuits and other components contained within the product being powered. Relative to the cost and the quality of the products in which they are used, such power supplies are expensive, bulky and inefficient. Efficiency is important to provide the equipment a long battery life. FIG. 1 shows a typical prior art power supply used in portable equipment powered by a battery 10. The signal from battery 10 is transmitted on lead 10a to a level translation circuit 12, which is controlled by a control signal from analog pulse width modulated controller 11. The control signal from analog pulse width modulator is responsive to the voltage drop across resistor 16 as detected by signals on conductive leads 17a and 17b connecting, respectively, the two terminals of resistor 16 into analog PWM controller 11. N-channel MOS transistors 13a and 13b are connected to operate in a complementary fashion. Level translation circuit 12 provides a high level voltage to the gate of N-channel transistor 13a to apply a pulse from battery 10 to one input terminal of coil 15. The other input terminal of coil 15 is connected to one terminal of resistor 16. The other terminal of resistor 16 is connected to load capacitor 18, which contains a charge at the voltage necessary to supply the particular circuitry being powered by this portion of the power supply. The analog PWM controller 11 measures the current through resistor 16 and controls the ON time of N-channel MOS transistor 13a. N-channel MOS transistor 13b is driven by the complement of the signal driving the gate of N-channel MOS transistor 13a and turns on to pull the input lead of coil 15 to ground and to shut off the current required to be supplied through resistor 16 to the power supply. Internal circuitry of analog pulse width controller 11 is shown schematically in FIG. 2.
As shown in FIG. 2, current source 20 provides a charging current to capacitor 21 to generate a ramp voltage across this capacitor. This ramp voltage is provided to the positive input lead of differential amplifier 22a, the negative input lead of which receives the output signal from differential amplifier 22b. The positive input lead of amplifier 22b is connected to the load capacitor 18 and carries a signal representing the voltage across the load capacitor 18. The negative input lead of differential amplifier 22b is connected to the node between resistors 23a and 23b making up a voltage divider (one terminal of which is connected to a reference voltage VRef and the other terminal of which is connected to the output lead of differential amplifier 22b). Thus when the output voltage across capacitor 18 is less than the voltage at node A between resistor 23a and resistor 23b, the output voltage from differential amplifier 22b goes to a low level. This low level output voltage is provided to the negative input lead of amplifier 22a, causing amplifier 22a to produce a positive output pulse. This positive output pulse is transferred to coil to provide a charging current to capacitor 18. With time, the charge on capacitor 18 increases until the voltage across capacitor 18 exceeds the voltage on node A. At this point the output voltage from differential amplifier 22b goes to a high level, so that the voltage at the negative input lead of differential amplifier 22a exceeds the voltage on the positive input lead of differential amplifier 22a, causing the output voltage from amplifier 22a to go a low level, and thus preventing further charging of capacitor 18. The voltage across coil 15 is negative, reflecting the negative rate of change in current in response to the trailing edge of the pulse from amplifier 22a going from a high level to a low level. The current through coil 15 does not change instantaneously due to the magnetic field of the coil but rather gradually changes with time. This type of power supply, which is characterized by a current source driving a capacitor, is known as an analog buck converter. Each MOSFET modulation cycle is formed by the precision comparator and the error amplifier. Such a power supply is difficult to scale and integrate into an integrated circuit and is typically fabricated using dedicated analog process technologies at captive semiconductor foundries.
Accordingly, what is needed is a power supply which provides different level precision voltages and at the same time and is simple to implement with a smaller number of components than in the prior art. Such a power supply must also be relatively inexpensive, robust and reliable.